High fluence particle beams must be generated under vacuum or near-vacuum conditions. That is, major mechanisms for generating, ampere-to-kiloampere level particle beams must be immersed in a vacuum environment, with background gas pressures of, e.g. 0.0001 Torr or less. To transmit the particle beam to the open atmosphere or into a gas background at atmospheric pressure, it is necessary to pass the beam through some form of "interface" or "window", which is both strong enough to withstand a 14.7 pounds-per-square-inch pressure differential and thin enough to allow passage of the beam's particles with minimum degradation of particle energy and with minimum scattering. To-date this has been accomplished through the use of various thin foils of materials such as aluminum, titanium, beryllium, diamond, sapphire or a high tensile strength plastic such as polyester, e.g. "Mylar" or polyimide, e.g. "Kapton". Such window can typically sustain a steady-state electron beam current density of 10 microamperes per cm.sup.2 at 150 keV before its ability to dissipate the heat influx is surpassed. Unfortunately, such low current densities limit practical applications. Electron-beam welding, for example, requires a minimum of 5 ma of continuous current. (For in-air welding, expensive pumping systems are used to transport the beam through a hole to the workpiece.). Thus, any attempt to pass a continuous beam of greater current density through existing foil windows results in a heating, softening, and rupturing of the foil window which destroys the vacuum environment necessary in the beam's generation region. Such failures can occur on microsecond timescales depending upon the specific foil material and energy deposition rate.
Attempts have been made in the prior art to cool such transmission window by circulating coolant through conduits proximate such window, see for example U.S. Pat. No. 5,235,239 to Jacob et al (1993), e.g. FIGS. 2 and 5A. In each case, coolant is circulated near a transmission window for indirect conductive cooling thereof through intervening structural members as shown, which limits the cooling effect thereof on such window. Also per FIG. 5A, the coolant system is located in a grid of support bars that block or cast shadows on the transmission window and absorb a significant portion of a particle beam passed therethrough.
There is thus a need for a cooling system for such transmission window that overcomes the above prior art shortcomings.
There has now been discovered a transmission window cooling system in which coolant directly cools the foils of such window, e.g. via convective heat transport with minimal absorption of the particle beam transmitted therethrough. At the same time per the invention, the coolant system is enclosed and pressurized to permit circulation of coolant therethrough for more effective cooling of such window.